Method of forming insulating film and method of fabricating semiconductor device including plasma bias for forming a second insulating film

ABSTRACT

With keeping an atmosphere including oxygen within a chamber and with a wafer kept at a low temperature, plasma generated within the chamber is biased toward the wafer, and the wafer is subjected to the plasma. A semiconductor layer exposed on the wafer is oxidized into an oxide film. Thus, an oxide film can be formed even at room temperature differently from thermal oxidation. This oxidation is applicable to recovery of an implantation protection insulating film having been etched in cleaning a photoresist film, relaxation of a step formed between polysilicon films, relaxation of a step formed within a trench and the like. Also, before removing a photoresist film used for forming a gate electrode including a metal, a contamination protection film can be formed by this oxidation with the photoresist film kept.

BACKGROUND OF THE INVENTION

The present invention relates to a method of forming an insulating filmin which a highly reliable oxide film can be formed at a low temperatureand a method of fabricating a semiconductor device by utilizing themethod of forming an insulating film.

In accordance with recent demands for high integration of semiconductorintegrated circuits, for example, a very shallow junction structure isemployed in forming a transistor and an STI (shallow trench isolation)structure is employed in forming an isolation. Since the very shallowjunction structure and the STI are thus employed, dislocation defectsare caused in an active region due to stress collected on the edge ofthe STI during formation of a gate oxide film (by thermal oxidation). Asa result, junction leakage can be increased, or variation in thethreshold voltage can be increased owing to change of junction profilecaused in the formation of the gate oxide film. Therefore, in order toovercome these problems, it is very significant to conduct the processfor forming an oxide film at a low temperature.

Also, in accordance with the demands for high integration ofsemiconductor integrated circuits, the gate length of a MOSFET isreduced, which makes it difficult to suppress the short channel effect.Therefore, the short channel effect is suppressed by employing a gateelectrode structure designated as a dual gate electrode obtained byimplanting phosphorus ions into a polysilicon film for a gate electrodeof an NMOSFET and implanting boron ions into a polysilicon film for agate electrode of a PMOSFET.

FIGS. 21( a) through 21(d) and 22(a) through 22(d) are sectional viewsfor showing procedures in fabrication of a conventional CMOS devicehaving a trench isolation structure and a dual gate electrode structure.

First, in the procedure shown in FIG. 21( a), a trench isolation region101 is formed in a Si substrate 100, and then, a photoresist film 103covering an NMOSFET formation region Rn and having an opening on aPMOSFET formation region Rp is formed on a protection oxide film 102 byphotolithography. Thereafter, phosphorus ions (P⁺) for forming an N-typewell region 104, phosphorus ions (P⁺) for controlling a thresholdvoltage and arsenic ions (As⁺) for stopping punch-through are implantedinto a region of the Si substrate 100 within the opening of thephotoresist film 103 (namely, the PMOSFET formation region Rp).

Then, in the procedure shown in FIG. 21( b), the photoresist film 103 isremoved by ashing and RCA cleaning.

Next, in the procedure shown in FIG. 21( a), a photoresist film 105covering the PMOSFET formation region Rp and having an opening on theNMOSFET formation region Rn is formed on the protection oxide film 102by the photolithography. Thereafter, boron ions (B⁺) for forming aP-type well region 106, boron ions (B⁺) for controlling a thresholdvoltage and boron ions (B⁺) for stopping punch-through are implantedinto a region of the Si substrate 100 within the opening of thephotoresist film 105 (namely, the NMOSFET formation region Rn).

Then, in the procedure shown in FIG. 21( d), the photoresist film 105 isremoved by the ashing and the RCA cleaning, and the protection oxidefilm 102 is also removed. Thereafter, the Si substrate 100 is heated atapproximately 800 through 1000° C. in an oxygen atmosphere, therebyforming gate oxide films 107 a and 107 b on the N-type well region 104and the P-type well region 106, respectively.

Subsequently, in the procedure shown in FIG. 22( a), after depositing apolysilicon film 108 for a gate electrode on the substrate, aphotoresist film 109 covering the NMOSFET formation region Rn and havingan opening on the PMOSFET formation region Rp is formed on thepolysilicon film 108. Thereafter, boron ions (B⁺) are implanted into aregion of the polysilicon film within the opening of the photoresistfilm 109 (namely, the PMOSFET formation region Rp).

Similarly, in the procedure shown in FIG. 22( b), after removing thephotoresist film 109 by the ashing and the RCA cleaning, a photoresistfilm 110 covering the PMOSFET formation region Rp and having an openingon the NMOSFET formation region Rn is formed on the polysilicon film 108by the photolithography. Thereafter, phosphorus ions (P⁺) are implantedinto a region of the polysilicon film 108 within the opening of thephotoresist film 110 (namely, the NMOSFET formation region Rn).

Next, in the procedure shown in FIG. 22( c), the photoresist film 110 isremoved by the ashing and the RCA cleaning, and then, a heat treatmentis carried out for activating the impurities implanted into thepolysilicon film 108. In this manner, a P-type polysilicon film 108 p isformed in the PMOSFET formation region Rp and an N-type polysilicon film108 n is formed in the NMOSFET formation region Rn.

Then, the P-type polysilicon film 108 p and the N-type polysilicon film108 n are respectively patterned into a gate electrode 108 a of thePMOSFET and a gate electrode 108 b of the NMOSFET.

Furthermore, in order to cope with reduction in a chip area and highoperation speed of a device, the resistance of the gate electrode of aMOSFET has recently been lowered. As one of promising means for loweringthe resistance, the so-called polymetal gate structure or polycide gatestructure in which part of the gate electrode is formed from a metal(refractory metal or its silicide) is known.

FIGS. 23( a) through 23(d) are sectional views for showing procedures infabrication of a conventional CMOS device having the polymetalstructure.

First, through the same procedures as those shown in FIGS. 21( a)through 21(d), a trench isolation region 101 for isolating a PMOSFETformation region Rp and an NMOSFET formation region Rn from each other,an N-type well region 104, a P-type well region 106 and gate oxide films107 a and 107 b are formed in a Si substrate 100. Thereafter, as isshown in FIG. 23( a), a polysilicon film 120, a metal film 121 oftitanium silicide or the like and an insulating film 122 of a siliconnitride film or the like are successively deposited on the substrate.

Next, in the procedure shown in FIG. 23( b), a photoresist film 115covering a gate electrode formation region is formed by thephotolithography, and then, dry etching (anisotropic etching) is carriedout by using the photoresist film as a mask, thereby patterning theinsulating film 122, the metal film 121 and the polysilicon film 120. Inthis manner, a gate electrode 125 a including a bottom gate electrode120 a and a top gate electrode 121 a, and an over-gate protection film122 a are formed in the PMOSFET formation region Rp. Also, a gateelectrode 125 b including a bottom gate electrode 120 b and a top gateelectrode 121 b, and an over-gate protection film 122 b are formed inthe NMOSFET formation region Rn.

Then, in the procedure shown in FIG. 23( c), a photoresist film 116covering the NMOSFET formation region Rn and having an opening on thePMOSFET formation region Rp is formed on the substrate. Thereafter,boron ions (B⁺) are implanted into the Si substrate 100 by using thephotoresist film 116 and the gate electrode 125 a as masks, therebyforming source/drain regions 126 of the PMOSFET.

Next, in the procedure shown in FIG. 23( d), the photoresist film 116 isremoved by the ashing and the RCA cleaning, and then, a photoresist film(not shown) covering the PMOSFET formation region Rp and having anopening on the NMOSFET formation region Rn is formed on the substrate.Thereafter, arsenic ions (As⁺) are implanted into the Si substrate 100by using the photoresist film and the gate electrode 125 b as masks,thereby forming source/drain regions 127 of the NMOSFET. Then, thephotoresist film is removed by the ashing and the RCA cleaning.

The conventional semiconductor devices fabricated as described abovehave, however, the following problems:

First, as is shown in FIG. 22( d), the gate oxide film 107 a of thePMOSFET and the gate oxide film 107 b of the NMOSFET have differentthicknesses. This is because, in the thermal oxidation for forming thegate oxide films in the procedure of FIG. 21( d), the oxidizing rate ishigher in the portion of the protection oxide film 102 corresponding tothe NMOSFET formation region Rn where the boron ions are implanted thanin the portion thereof corresponding to the PMOSFET formation region Rpwhere the phosphorus (or arsenic) ions are implanted. Also, since theimpurity concentration profile in the P-type well region 106 forcontrolling the threshold voltage of the NMOSFET and the impurityconcentration profile in the N-type well region 104 for controlling thethreshold voltage of the PMOSFET are changed in the heat treatmentconducted at 850 through 1000° C., the short channel effect of theMOSFETs are accelerated, variation in the threshold voltage of theNMOSFET and the PMOSFET is increased, and an off leakage current isincreased.

Secondly, the boron implanted into the P-type polysilicon film 108 p ofthe polysilicon film 108 for the gate electrode is diffused into theN-type well region 104 through the gate oxide film 107 a due to the heattreatment conducted at 900 through 1000° C. for the thermal oxidation.As a result, the reliability of the gate oxide film is degraded, andvariation in the threshold voltage of the PMOSFET is increased.

Thirdly, as is shown in FIG. 21( b), when the photoresist film 103 isremoved by the ashing and the RCA cleaning after the ion implantation,the surface of the protection oxide film 102 becomes very rough. This isprobably because the protection oxide film 102 is damaged by the ionsduring the ion implantation and is ununiformly etched by the RCAcleaning. When the ion implantation for forming the well region, namely,for controlling the threshold voltage, is carried out with theprotection oxide film 102 having a very rough surface, the impurityconcentration in a portion corresponding to a channel region within thewell region is largely varied among MOSFETs. In this manner, variationin the threshold voltage among the MOSFETs is increased. Furthermore,the Si substrate 100 is also etched by the RCA cleaning. For example,when the RCA cleaning is carried out with the ion-implanted Si substrateexposed, a portion of the Si substrate 100 where the impurity ions havebeen implanted for controlling threshold voltage may also be etched by athickness of several nm. As a result, the concentration profile of theimplanted impurity is changed, so that the threshold voltage is largelyvaried in, particularly, a MOSFET having a buried transistor structure.

Fourthly, as is shown in FIG. 22( d), in patterning the polysilicon film108 into the gate electrodes 108 a and 108 b, the surface of the activeregion of the Si substrate 100 can be roughened. Even when the etchingend point of the polysilicon film is detected, the polysilicon film isnot completely removed but partly remains as etching residues orsidewalls. Therefore, in order to remove the remaining portions of thepolysilicon film, the polysilicon film is over-etched. Due to recentdecrease in the thickness of a gate oxide film (to several nm), however,merely a portion of the gate oxide film not covered with the polysiliconfilm can be etched before completely removing the polysilicon filmthrough the over-etching. Accordingly, when the Si substrate 100 belowis partly etched, the surface of the active region is roughened. As aresult, a good silicide layer cannot be formed in a salicidationprocess. Furthermore, the profile of the implanted ions for forming thesource/drain regions cannot be uniform, resulting in increasing junctionleakage.

Fifthly, as is shown in FIG. 23( d), in removing the photoresist film116 by the ashing and the RCA cleaning after patterning the metal film121, the top electrodes 121 a and 121 b, which are made from a metal inthe gate electrodes 125 a and 125 b of the MOSFETS, are etched on theirside faces. When metal ions dissolved in the etching solution (cleaningsolution) enter the active region through the surface of the Sisubstrate 100, junction leakage is caused in the MOSFET. On the otherhand, when a thermal oxide film for covering the substrate surface isformed to prevent this contamination, the top electrodes 121 a and 121 bformed from the metal are peeled.

-   -   Sixthly, as is shown in FIG. 22( b), in removing the photoresist        films 109 and 110 by the ashing and the RCA cleaning or in        conducting cleaning before loading the substrate in a furnace,        the polysilicon film 108 is etched to some extent. Since the        P-type polysilicon film 108 p where the boron ions are implanted        and the N-type polysilicon film 108 n where the phosphorus (or        arsenic) ions are implanted have different etch rates, there may        be a step on the boundary between the P-type polysilicon film        108 p and the N-type polysilicon film 108 n. When this step is        abrupt, although no problem can be observed in the sectional        view shown in FIG. 22( d), the following problem may occur in a        CMOS inverter having a silicide gate structure:        -   FIGS. 24( a) through 24(c) are sectional views in a            silicidation process for showing the gate electrodes 108 a            and 108 b alone taken on line perpendicular to the section            of FIG. 22( d) (namely, line XXIV—XXIV of FIG. 25).            Furthermore, FIG. 25 is a plan view of the gate electrodes            and a portion below the gate electrodes of the CMOS            inverter. In this manner, in the CMOS inverter, the gate            electrodes of the PMOSFET and the NMOSFET are mutually            connected in the section perpendicular to the section of            FIG. 22( d).

In the case where the abrupt step as shown in FIG. 24( a) is present,even when, for example, a Co film is deposited on the gate electrodes108 a and 108 b for forming a silicide film on the gate electrodes 108 aand 108 b in a later procedure, the Co film cannot be sufficientlydeposited on the side face of the step.

As a result, as is shown in FIG. 24( c), merely a very thin silicidefilm of CoSi₂ or the like is formed or no silicide film is formed on thestep through the silicidation. Accordingly, even when a voltage isapplied to the gate electrode 108 b of the NMOSFET in the CMOS inverter,the resistance between the gate electrodes can be too large to transferthe electric field to the gate electrode 108 a of the PMOSFET.

Seventhly, the following problem occurs in forming the STI structure(trench isolation region). FIG. 26 is a sectional view for showing theshape of a conventional trench isolation region. As is shown in FIG. 26,a pad oxide film 131 and a masking nitride film 132 are stacked on a Sisubstrate 100, and a portion of the Si substrate 100 below an opening ofthe masking nitride film 132 is etched so as to form a trench 134. Then,a thermal oxide film 135 is formed by thermally oxidizing a portion ofthe Si substrate 100 within the trench, and the trench is filled with aCVD oxide film, so as to form a trench isolation region 136.

The thickness of the thermal oxide film 135 is, however, varied atrespective edges within the trench depending upon the thickness of themasking nitride film 132, the thickness of the pad oxide film 131 or theplane size of the masking nitride film 132. In particular, when a honephenomenon in which the thermal oxide film 135 has a small thickness atan edge is caused, an abrupt edge is formed at the corresponding cornerof the Si substrate 100 within the trench 134. As a result, the electricfiled is collected on the edge so as to cause problems such as breakdownof a gate insulating film and a hump characteristic (actuation of anedge transistor). The hone characteristic is conspicuous particularlywhen the thermal oxide film 135 is formed at a low temperature of 900°C. or less. Therefore, the temperature of the thermal oxidation can beset to 1000° C. for avoiding the hone phenomenon, but as the temperatureof the thermal oxidation increases, larger stress is caused in thenitride film 132, resulting in increasing defects occurring in the Sisubstrate 100.

SUMMARY OF THE INVENTION

An object of the invention is, in considering that the aforementionedproblems are basically derived from a high temperature required forforming an oxide film through thermal oxidation, providing means forforming an oxide film through oxidation conducted at a low temperature,so as to provide a method of forming an insulating film and a method offabricating a semiconductor device in which the aforementioned problemscan be overcome.

In order to overcome the problems, an oxide film or a nitrided oxidefilm is formed at a low temperature by utilizing biased plasma in thisinvention.

The method of this invention of forming an insulating film for asemiconductor device for forming, on a semiconductor layer exposed on asubstrate, the insulating film through a reaction between at leastoxygen and a semiconductor, comprises the steps of (a) loading thesubstrate including the semiconductor layer in a processing chamber; and(b) generating, within the processing chamber, plasma biased toward thesubstrate with the processing chamber kept in an atmosphere includingoxygen, and subjecting the semiconductor layer to the biased plasma.

In this method, an insulating film can be formed through oxidation of asemiconductor using plasma and conducted at a temperature lower than inthermal oxidation. Accordingly, by utilizing this characteristic,insulating films functioning as various members of a semiconductordevice can be formed while avoiding the problems such as thecharacteristic degradation derived from subjecting the substrate to ahigh temperature.

In the method of forming an insulating film, a thickness of theinsulating film can be controlled by adjusting magnitude of a degree ofbiasing the plasma in the step (b).

In the method of forming an insulating film, the step (b) is preferablycarried out at a temperature of 300° C. or less.

In the method of forming an insulating film, the step (b) is morepreferably carried out at a temperature of 200° C. or less.

In the method of forming an insulating film, the step (b) can be carriedout with a photoresist film formed on the substrate.

In the method of forming an insulating film, the insulating film can beused as a gate insulating film of a MIS transistor.

The method of forming an insulating film can further comprise, before atleast the step (b), a step of forming a first active region doped withan impurity of a first conductivity type and a second active regiondoped with an impurity of a second conductivity type, and a firstinsulating film and a second insulating film can be respectively formedon the first active region and the second active region in the step (b).In this manner, differently from the thermal oxidation, the firstinsulating film and the second insulating film can be formed insubstantially the same thickness.

The method of forming an insulating film can further comprise, after thestep (b), a step of conducting a heat treatment on the insulating film.In this manner, the insulating film can be made in uniform quality andcarbon contamination thereon can be removed, resulting in improving thereliability of the insulating film.

In the method of forming an insulating film, the step (b) can be carriedout in an atmosphere including nitrogen and oxygen, in an atmosphereincluding a NO gas (a nitriding oxidation atmosphere), or in anatmosphere including oxygen and N₂ (a nitriding oxidation atmosphere).In this case, a nitrided oxide film is formed.

Alternatively, when the step (b) is carried out in an atmosphereincluding O₂ but substantially no nitrogen, an oxide film is formed.

The first method of fabricating a semiconductor device of this inventioncomprises the steps of (a) forming an insulating film on first andsecond active regions of a semiconductor substrate; (b) forming a firstphotoresist film covering the second active region and having an openingon the first active region; (c) implanting impurity ions into the firstactive region through the first photoresist film; (d) removing the firstphotoresist film; (e) recovering a thickness of the insulating film bysubjecting, in an atmosphere including oxygen, the semiconductorsubstrate to plasma biased toward the semiconductor substrate; (f)forming a second photoresist film covering the first active region andhaving an opening on the second active region; and (g) implantingimpurity ions into the second active region through the secondphotoresist film.

In this method, although the insulating film is also etched to causevariation in its thickness by the ion implantation and the ashing andcleaning conducted for removing the first photoresist film in the step(d), the thickness of the insulating film is recovered to asubstantially uniform thickness by the bias plasma oxidation conductedin the step (e). Accordingly, the distribution of the impurity ionsimplanted into the second active region in the following step (g) can becontrolled with good reproducibility.

In the first method of fabricating a semiconductor device, when the step(c) corresponds to impurity ion implantation for controlling a thresholdvalue of a MISFET, the variation in the threshold value of the MISFETcan be suppressed.

The second method of fabricating a semiconductor device of thisinvention comprises the steps of (a) forming a semiconductor film on asemiconductor substrate; (b) forming, on the semiconductor film, a firstphotoresist film covering a first part of the semiconductor film andhaving an opening on a second part of the semiconductor film adjacent tothe first part, and implanting impurity ions of a first conductivitytype into the semiconductor film through the first photoresist film; (c)after removing the first photoresist film, forming a second photoresistfilm covering the second part of the semiconductor film and having anopening on the first part, and implanting impurity ions of a secondconductivity type into the semiconductor film through the secondphotoresist film; (d) removing the second photoresist film; and (e)forming an insulating film on the semiconductor film through a reactionbetween at least oxygen and a semiconductor by subjecting, in anatmosphere including oxygen, the semiconductor substrate to plasmabiased toward the semiconductor substrate.

In this method, although a step is formed on the top face of thesemiconductor film in the ashing and the cleaning for removing thephotoresist film in the step (d) owing to the difference in theconductivity type of the impurities implanted in the semiconductor film,a portion in the vicinity of the top face of the semiconductor film isoxidized in the process conducted at a low temperature in the step (e),so as to round the abrupt step. Accordingly, the harmful effect of theabrupt step on members formed on the semiconductor film afterward can beavoided without harmfully affecting the impurity distribution in thesemiconductor film.

The second method of fabricating a semiconductor device can furthercomprise, before the step (a), a step of forming gate insulating filmsrespectively on a first conductivity type MISFET formation region and asecond conductivity type MISFET formation region of the semiconductorsubstrate, and the semiconductor film can be formed on the gateinsulating films over the first and second conductivity type MISFETformation regions in the step (a), the first part may correspond to thesecond conductivity type MISFET formation region and the second partcorresponds to the first conductivity type MISFET formation region inthe steps (b) and (c), and the method can further include, after thestep (d), a step of patterning the semiconductor film into a gateelectrode of a dual gate type over the first conductivity type MISFETformation region and the second conductivity type MISFET formationregion. In this case, an electric field can be well transferred betweenthe gate electrodes of a CMIS inverter.

The second method of fabricating a semiconductor device can furthercomprise, after at least the step (d), a step of siliciding an upperportion of the semiconductor film after removing at least part of athickness of the insulating film formed in the step (e) In this manner,an electric field can be well transferred between gate electrodes withlow resistance.

The third method of fabricating a semiconductor device of this inventioncomprises the steps of (a) forming an insulating film on a semiconductorsubstrate; (b) forming a semiconductor film on the insulating film; (c)forming a gate electrode of a MISFET by patterning the semiconductorfilm by etching with a photoresist film used as a mask; and (d)oxidizing etching residues of the semiconductor film remaining on theexposed insulating film by subjecting, in an atmosphere includingoxygen, the semiconductor substrate to plasma biased toward thesemiconductor substrate, with keeping said photoresist film.

In this method, etching residues functioning as a conductor can beprevented from remaining around a member formed by patterning thesemiconductor film, and the semiconductor substrate can be preventedfrom being roughened on its surface through ununiform etching inremoving the insulating film afterward.

The third method of fabricating a semiconductor device can furthercomprise, after the step (d), steps of removing the oxidized etchingresidues and an exposed portion of the insulating film; and silicidingpart of the semiconductor substrate exposed by removing the exposedportion of the insulating film. In this manner, a silicide layer withlow resistance can be formed as part of a source/drain region of aMISFET.

When the step (d) is carried out at a temperature of 200° C. or less,the step (d) can be carried out with the photoresist film kept.Thereafter, oxidized etching residues and an insulating film can beremoved by etching using the photoresist film.

The fourth method of fabricating a semiconductor device of thisinvention comprises the steps of (a) successively depositing a firstinsulating film and a conducting film at least including a metal on asemiconductor substrate; (b) patterning the conducting film and thefirst insulating film by etching with a photoresist film used as a maskinto a gate electrode and a gate insulating film; (c) forming a secondinsulating film on at least an exposed portion of the semiconductorsubstrate through a reaction between oxygen and a semiconductor bysubjecting, in an atmosphere including oxygen, the semiconductorsubstrate to plasma biased toward the semiconductor substrate, with thephotoresist film kept; (d) removing the photoresist film; and (e)forming source/drain regions by introducing an impurity into regionspositioned on both sides of the gate electrode in the semiconductorsubstrate.

In this method, even if the metal of the conducting film included in thegate electrode is dissolved in the cleaning solution, the metal ions canbe prevented from entering the semiconductor substrate because thesecond insulating film is present on the semiconductor substrate. Inaddition, since there is no need to conduct a high temperature treatmentlike the thermal oxidation, the metal of the conducting film can beavoided from being oxidized.

In the fourth method of fabricating a semiconductor device, apolysilicon film and a metal film stacked thereon can be formed as theconducting film in the step (a), a bottom electrode of a polysiliconfilm and a top electrode of a metal film can be formed as the gateelectrode in the step (b), and the second insulating film can be formedalso on side faces of the bottom electrode in the step (c). In thismanner, a semiconductor device including a gate electrode having thepolymetal structure or the polycide structure can be fabricated.

In the fourth method of fabricating a semiconductor device, a siliconnitride film can be further formed on the conducting film in the step(a), an over-gate protection film of a nitride film can be formed on thetop electrode in the step (b), and the method can further include, afterthe step (d), steps of (f) forming nitride film sidewalls on side facesof the polysilicon film and the metal film; (g) depositing an interlayerinsulating film of a silicon oxide film on the substrate; and (h)forming a contact hole penetrating through the interlayer insulatingfilm and reaching the source/drain region in a self-alignment manneragainst the gate electrode. In this manner, a semiconductor device,suitable to refinement, having the polymetal structure or the polycidestructure and the so-called SAC (self-aligned contact) structure can befabricated.

In the fourth method of fabricating a semiconductor device, the step (c)is preferably carried out at a temperature of 200° C. or less.

The fifth method of fabricating a semiconductor device of this inventioncomprises the steps of (a) forming a first gate electrode from asemiconductor film including an impurity of a first conductivity type ona first conductivity type MISFET formation region of a semiconductorsubstrate with a first gate insulating film sandwiched therebetween, andforming a second gate electrode from a semiconductor film including animpurity of a second conductivity type on a second conductivity typeMISFET formation region of the semiconductor substrate with a secondgate insulating film sandwiched therebetween; (b) forming a coatinsulating film through a reaction between at least oxygen and asemiconductor on the semiconductor substrate and exposed portions of thefirst and second gate electrodes by subjecting, in an atmosphereincluding oxygen, the semiconductor substrate to plasma biased towardthe semiconductor substrate; (c) forming source/drain regions of a firstconductivity type MISFET through ion implantation of an impurity of thefirst conductivity type by using, as masks, a first photoresist filmcovering the second conductivity type MISFET formation region and havingan opening on the first conductivity type MISFET formation region andthe first gate electrode; (d) removing the first photoresist film; and(e) forming source/drain regions of a second conductivity type MISFETthrough ion implantation of an impurity of the second conductivity typeby using, as masks, a second photoresist film covering the firstconductivity type MISFET formation region and having an opening on thesecond conductivity type MISFET formation region and the secondgate-electrode.

In this method, since the coat insulating film can be formed at a lowtemperature in the step (b), punch-through of boron included in the gateelectrode into the semiconductor substrate caused in the thermaloxidation can be avoided. Furthermore, since the surface of thesemiconductor substrate can be protected by the coat insulating film,the surfaces of the source/drain regions can be prevented from beingetched in removing the photoresist film in the following step (d) evenwhen the gate insulating film has a small thickness, and hence, thesheet resistance of the source/drain regions can be kept small.

In the fifth method of fabricating a semiconductor device, the step (b)is preferably carried out at a temperature of 300° C. or less.

Furthermore, the first photoresist film is preferably removed in thestep (d) by ashing with a degree of biasing plasma smaller than a degreeof biasing the plasma in the step (b). In this manner, the ashing can becarried out without increasing the thickness of the coat insulating filmformed on the semiconductor substrate. As a result, the impurity profilein the surface portion of the semiconductor substrate is minimallychanged, and hence, the leak characteristic and the like of thesemiconductor device can be kept satisfactorily.

The sixth method of fabricating a semiconductor device of this inventioncomprises the steps of (a) successively depositing a pad oxide film anda masking nitride film on a semiconductor substrate; (b) forming anopening in the masking nitride film and the pad oxide film in a positioncorresponding to a trench formation region; (c) forming a trench in thesemiconductor substrate by conducting etching with the masking nitridefilm used as a mask; (d) forming a rounding insulating film through areaction between at least oxygen and a semiconductor on a portion of thesemiconductor substrate exposed within the trench by subjecting, in anatmosphere including oxygen, the semiconductor substrate to plasmabiased toward the semiconductor substrate; and (e) forming a trenchisolation region by filling the trench with an insulating film.

In this method, an abrupt edge of the semiconductor substrate exposedbecause the pad oxide film sinks in forming a trench by the etching inthe step (b) can be rounded by forming the rounding insulating film inthe step (d). Therefore, it is possible to suppress the degradation ofthe reliability of the gate insulating film derived from electric fieldcollection in a MISFET formed therein and the occurrence of the humpcharacteristic in the MISFET.

The sixth method of fabricating a semiconductor device can furthercomprises, after the step (d) and before the step (e), a step ofincreasing a thickness of the rounding insulating film by thermaloxidation. In this manner, an underlying oxide film for a trenchisolation region can be formed without causing the problem of theelectric field collection and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view for schematically showing the structureof a bias plasma generation system used in each embodiment of theinvention;

FIGS. 2( a) and 2(b) show data of dependency, on processing time andbias, of the thickness of a silicon oxide film formed by bias plasmaoxidation;

FIG. 3 is a diagram for showing dependency, on the thickness of aninitial oxide film, of increase in the thickness of an oxide film formedby the bias plasma oxidation conducted on a wafer where the initialoxide film is previously formed;

FIGS. 4( a), 4(b), 4(c) and 4(d) are cross-sectional views for showingprocedures in fabrication of a CMOS device having a trench isolationstructure and a dual gate structure according to Embodiment 1 up to ionimplantation for forming wells;

FIGS. 5( a), 5(b), 5(c) and 5(d) are cross-sectional views for showingother procedures in the fabrication of the CMOS device having the trenchisolation structure and the dual gate structure of Embodiment 1 up toimpurity ion implantation into a polysilicon film for gate electrodes;

FIGS. 6( a), 6(b), 6(c) and 6(d) are cross-sectional views for showingother procedures in the fabrication of the CMOS device having the trenchisolation structure and the dual gate structure of Embodiment 1 up toformation of the gate electrodes;

FIGS. 7( a), 7(b), 7(c) and 7(d) are cross-sectional views for showingother procedures in the fabrication of the CMOS device having the trenchisolation structure and the dual gate structure of Embodiment 1 up toformation of a salicide film;

FIGS. 8( a), 8(b), 8(c), 8(d) and 8(e) are TEM images of oxide filmsformed through the bias plasma oxidation on a channel region of anNMOSFET, a channel region of a PMOSFET, a substantially intrinsicsubstrate, an N-type polysilicon film and a P-type polysilicon film,respectively;

FIG. 9 is a diagram for showing the result of QBD evaluation of aPMOSFET including a gate insulating film formed through the bias plasmaoxidation in Embodiment 1;

FIG. 10 is a diagram for showing the result of the QBD evaluation of aconventional PMOSFET including a gate insulating film of a thermal oxidefilm (pyrogenic oxidized at 900° C.);

FIGS. 11( a), 11(b), 11(c), 11(d) and 11(e) are cross-sectional views ofthe structure of the gate electrodes alone in the bias plasma oxidationand silicidation taken on a section perpendicular to the section of FIG.6( a);

FIGS. 12( a) and 12(b) are respectively a diagram obtained throughthree-dimensional observation with AFM of a step formed on the surfacesof N-type and P-type polysilicon films after ion implantation and adiagram of minute steps in a section of the N-type and P-typepolysilicon films after the ion implantation;

FIG. 13 is a diagram for showing difference in an electric resistancevalue of a polycide layer resulting from the bias plasma oxidation;

FIGS. 14( a), 14(b), 14(c) and 14(d) are cross-sectional views forshowing first half procedures in fabrication of a CMOS device having apolymetal gate structure and the like according to Embodiment 2;

FIGS. 15( a), 15(b) and 15(c) are cross-sectional views for showingsecond half procedures in the fabrication of the CMOS device having thepolymetal gate structure and the like of Embodiment 2;

FIG. 16 is a cross-sectional view of a CMOS device having a SACstructure according to modification of Embodiment 2;

FIGS. 17( a), 17(b) and 17(c) are cross-sectional views for showingfirst half procedures in fabrication of a CMOS device having a salicidestructure according to Embodiment 3;

FIGS. 18( a), 18(b) and 18(c) are cross-sectional views for showingsecond half procedures in the fabrication of the CMOS device having thesalicide structure of Embodiment 3;

FIGS. 19( a) and 19(b) show data for comparison in the Ion-Ioffcharacteristic between MOSFETs formed in Embodiment 3 and MOSFETs formedwithout the plasma oxidation;

FIGS. 20( a), 20(b) and 20(c) are cross-sectional views for showing partof procedures for forming a trench isolation region of a semiconductordevice according to Embodiment 4;

FIGS. 21( a), 21(b), 21(c) and 21(d) are cross-sectional views forshowing first half procedures in fabrication of a conventional CMOSdevice having a trench isolation structure and a dual gate structure;

FIGS. 22( a), 22(b), 22(c) and 22(d) are cross-sectional views forshowing second half procedures in the fabrication of the conventionalCMOS device having the trench isolation structure and the dual gatestructure;

FIGS. 23( a), 23(b), 23(c) and 23(d) are cross-sectional views forshowing procedures in fabrication of a conventional CMOS device having apolymetal structure;

FIGS. 24( a), 24(b) and 24(c) are cross-sectional views of the structureof gate electrodes alone in silicidation taken on a sectionperpendicular to the section of FIG. 22( d);

FIG. 25 is a plan view of gate electrodes and a portion below in aconventional CMOS inverter, that is, a semiconductor device; and

FIG. 26 is a cross-sectional view for showing the shape of aconventional trench isolation region.

DETAILED DESCRIPTION OF THE INVENTION

Formation of Oxide Film by Bias Plasma Oxidation

Before describing preferred embodiments of the invention, a systememployed for bias plasma oxidation of this invention and thecharacteristic of an oxide film formed by the bias plasma oxidation willbe described.

FIG. 1 is a sectional view for schematically showing the structure ofthe bias plasma generation system employed in each of the preferredembodiments. The bias plasma generation system includes a lowerelectrode 2 serving as an anode and disposed on the bottom of a chamber1, a bias electrode 3 serving as a cathode and opposing the lowerelectrode 2, and a high frequency power supply 5 for applying highfrequency power (of 13.56 MHz) to the lower electrode through acapacitor 6. A processing wafer 4 is placed on the lower electrode 2,plasma and a reaction gas (oxygen) are introduced through an upperportion of the chamber 1, and the reaction gas is vacuated through anexhaust port provided to the chamber 1 in the vicinity of the lowerelectrode 2. As the plasma generation system, any of various plasmageneration systems, such as a capacity coupling plasma system, aninduction coupling plasma system, an ECR plasma generation system and ahelicon plasma generation system, can be employed by additionallyproviding a bias electrode.

In conducting processing by using bias plasma, for example, thetemperature of the lower electrode 2 is set to 180° C., an oxygen gas isintroduced through the upper portion of the chamber at a flow rate ofapproximately 800 sccm, the gas pressure is set to 0.5 Torr (66.65 Pa)and high frequency power of 1000 W is applied by the high frequencypower supply 5. Thus, a Si layer (monosilicon, polysilicon or amorphoussilicon) exposed on the wafer 4 is oxidized into a silicon oxide film.The high frequency power can be replaced with a DC voltage. Theexperiment results described below are obtained by applying not the highfrequency power but the DC voltage.

FIGS. 2( a) and 2(b) show data of dependency, on processing time andbias, of the thickness of a silicon oxide film formed by conducting thebias plasma oxidation on a processing wafer. In FIG. 2( a), the abscissaindicates time (sec.) for applying the bias plasma, and the ordinateindicates the thickness (nm) of the formed silicon oxide film (SiO₂film). As is shown in FIG. 2( a), the bias plasma oxidation is found tohave a characteristic form of oxidation as follows: As the bias plasmaoxidation is proceeded, the thickness of the oxide film is abruptlyincreased to 3 nm at the initial stage, but thereafter, even though thebias plasma oxidation is further proceeded, the increasing rate of thethickness of the silicon oxide film is lowered. When the bias plasmaoxidation is carried out for 10 min., the thickness of the oxide film issubstantially saturated at approximately 6 nm.

FIG. 3 shows the dependency, on the thickness of an initial oxide film,of the increase in the thickness of an oxide film formed through thebias plasma oxidation conducted on a wafer where the initial oxide filmis previously formed. In FIG. 3, the abscissa indicates the thickness ofthe initial oxide film formed on the wafer by thermal oxidation beforeconducting the bias plasma oxidation, and the ordinate indicates theincrease (nm) in the thickness of the oxide film obtained by conductingthe bias plasma oxidation for 5 min. under the aforementioned conditions(with the temperature of the lower electrode set to 180° C.), namely,the thickness of a newly formed oxide film. As is shown in FIG. 3, inthe case where the initial oxide film has a thickness of 6 nm or less,the newly oxide film is formed through the bias plasma oxidationconducted for 10 min., so that the total thickness of the oxide filmscan be approximately 6 nm. In other words, the total thickness of theoxide films is naturally controlled to be a constant value (ofapproximately 6 nm). On the other hand, when the initial oxide film hasa thickness exceeding 6 nm, the total thickness of the oxide films isminimally increased through the bias plasma oxidation.

In FIG. 2( b), the abscissa indicates RF power (W) corresponding to thedegree of biasing the plasma, and the ordinate indicates the thickness(nm) of the formed silicon oxide film. As is shown in FIG. 2( b), thethickness of the oxide film is substantially linearly increased inaccordance with the RF power (bias). Specifically, the saturationthickness of the oxide film is largely affected by the bias, and it isconfirmed that as the bias increases, the saturation thickness of theoxide film increases and that as the bias decreases, the saturationthickness of the oxide film decreases. In other words, the totalthickness of the oxide films can be controlled in accordance with thebias.

As a characteristic of the bias plasma oxidation, an oxide film can besufficiently formed at a low temperature of 200° C. or less (includingroom temperature). In conducting the bias plasma oxidation at such a lowtemperature, even when a photoresist film is formed on the wafer, therate of removing the photoresist film is so low that it is minimallyremoved. This is because ashing for removing a photoresist film isgenerally conducted at approximately 250° C. or more. Accordingly, whena photoresist film is present on the wafer, the bias plasma oxidation ispreferably conducted at 200° C. or less, whereas when no photoresistfilm is present, the temperature can be increased to approximately 300°C.

Conventional plasma oxidation is carried out at 350 through 600° C., andtherefore, when a photoresist film is present on a wafer, thephotoresist film is unavoidably damaged in forming an oxide film.

The frequency of the high frequency power may be varied in a rangebetween 200 KHz and 20 MHz or the high frequency power may be a DCvoltage as described above, whereas the plasma can be biased probablymore effectively by using the high frequency power. In particular,radicals or ions included in the plasma can be more easily biased byusing high frequency power with a comparative low frequency of 800 KHzor 400 KHz.

The reaction gas is not limited to oxygen but can be a mixture ofnitrogen and oxygen. For example, the bias plasma oxidation can beconducted in a NO gas atmosphere or an atmosphere including oxygen andnitrogen. It goes without saying that it can be conducted in anatmosphere including oxygen but substantially no nitrogen.

Now, a method of forming a bias plasma oxide film and a method offabricating a semiconductor device using the method will be described.

EMBODIMENT 1

FIGS. 4( a) through 4(d), 5(a) through 5(d), 6(a) through 6(d) and 7(a)through 7(d) are sectional views for showing procedures in fabricationof a CMOS device having a trench isolation structure and a dual gatestructure according to Embodiment 1.

First, in the procedure shown in FIG. 4( a), after forming a trenchisolation region 12 in a Si substrate 11, a protection oxide film 13with a thickness of approximately 10 nm is formed in an active region ofthe Si substrate 11 by thermal oxidation. Then, a photoresist film Pr1covering an NMOSFET formation region Rn and having an opening on aPMOSFET formation region Rp is formed on the substrate byphotolithography. Thereafter, phosphorus ions (P⁺) are implanted into aportion of the Si substrate 11 within the opening of the photoresistfilm Pr1 (namely, the PMOSFET formation region Rp) at implantationenergy of 140 keV and a dose of 1×10¹²cm⁻², thereby forming an N-typewell region 15. Also, boron ions (B⁺) for controlling a thresholdvoltage are implanted at implantation energy of 20 keV and a dose of6×10¹² cm⁻², and arsenic ions (As⁺) for stopping punch-through areimplanted at implantation energy of 300 keV and a dose of 4×10¹² cm⁻².In this manner, the so-called buried channel region is formed.Alternatively, when a surface PMOSFET is desired, the ion implantationfor controlling the threshold voltage is carried out by implantingphosphorus ions (P⁺) at implantation energy of 50 keV and a dose of5×10¹² cm⁻².

Next, in the procedure shown in FIG. 4( b), the photoresist film Pr1 isremoved by ashing and RCA cleaning. Specifically, the photoresist filmis removed and the substrate is cleaned by ashing utilizing plasma in anoxygen atmosphere and by cleaning utilizing a mixed solution of sulfuricacid and hydrogen peroxide or hydrofluoric acid. At this point, theprotection oxide film 13 is etched mainly by the RCA cleaning, and henceits thickness is reduced as a whole and becomes ununiform. Therefore, inthe procedure shown in FIG. 4( c), the bias plasma oxidation is carriedout for 5 minutes by using the plasma generation system of FIG. 1 in anatmosphere including oxygen at a substrate temperature of 180° C. andbias power of 1000 W. Through this bias plasma oxidation, the protectionoxide film 13 is recovered into a protection oxide film 13 a with asubstantially uniform thickness of approximately 10 nm. Specifically, asis understood from FIGS. 2( a), 2(b) and 3, since an oxide film formedthrough the bias plasma oxidation has a thickness corresponding to aconstant saturation value determined depending upon the bias power, thedamaged protection oxide film can be thus recovered into the protectionoxide film 13 a with a substantially uniform thickness. This bias plasmaoxidation can be carried out at a high temperature of approximately 300°C.

Next, in the procedure shown in FIG. 4( d), a photoresist film Pr2covering the PMOSFET formation region Rp and having an opening on theNMOSFET formation region Rn is formed on the substrate by thephotolithography. Thereafter, boron ions (B⁺) are implanted into aportion of the Si substrate 11 within the opening of the photoresistfilm Pr2 (namely, the NMOSFET formation region Rn) at implantationenergy of 280 keV and a dose of 1×10¹³ cm⁻², thereby forming a P-typewell region 16. Also, boron ions (B⁺) for controlling a threshold valueare implanted at implantation energy of 30 keV and a dose of 6×10¹²cm⁻². Thus, the so-called surface type channel region is formed.

Subsequently, in the procedure shown in FIG. 5( a), the photoresist filmPr2 is removed by the ashing and the RCA cleaning, and the protectionoxide film 13 a is also removed. Then, the bias plasma oxidation iscarried out for 5 minutes in an atmosphere including oxygen (or oxygenand nitrogen) at a substrate temperature of 180° C. and bias power of1000 W. Thus, gate insulating films 17 a and 17 b of an oxide film (or anitrided oxide film) with a thickness of approximately 6 nm are formedin the active region of the Si substrate 11. At this point, theinsulating film formed through the bias plasma oxidation has a thicknesssaturated at a substantially constant value regardless of the kinds ofimpurities included in the Si layers (well regions 15 and 16) below.Accordingly, the gate insulating films 17 a and 17 b with an equivalentthickness of approximately 6 nm can be formed.

Next, in the procedure shown in FIG. 5( b), a polysilicon film 18 forgate electrodes with a thickness of approximately 200 nm is deposited onthe substrate, and then, a photoresist film Pr3 covering the NMOSFETformation region Rn and having an opening on the PMOSFET formationregion Rp is formed on the polysilicon film 18. Thereafter, boron ions(B⁺) are implanted into a portion of the polysilicon film 18 within theopening of the photoresist film Pr3 (namely, the PMOSFET formationregion Rp) at implantation energy of 5 keV and a dose of 3×10¹⁵ cm⁻².

Similarly, in the procedure shown in FIG. 5( c), the photoresist filmPr3 is removed by the ashing and the RCA cleaning, and a photoresistfilm Pr4 covering the PMOSFET formation region Rp and having an openingon the NMOFET formation region Rn is formed on the polysilicon film 18by the photolithography. Then, phosphorus ions (P⁺) are implanted into aportion of the polysilicon film 18 within the opening of the photoresistfilm Pr4 (namely, the NMOSFET formation region Rn) at implantationenergy of 15 keV and a dose of 5×10¹⁵ cm⁻².

Then, in the procedure shown in FIG. 5( d), the photoresist film Pr4 isremoved by the ashing and the RCA cleaning, and a heat treatment iscarried out for activating the impurities implanted into the polysiliconfilm 18. In this manner, a P-type polysilicon film 18 p is formed in thePMOSFET formation region Rp and an N-type polysilicon film 18 n isformed in the NMOSFET formation region Rn. At this point, the P-typepolysilicon film 18 p and the N-type polysilicon film 18 n are etched bythe RCA cleaning and cleaning conducted before loading the substrate ina furnace, and due to a difference therebetween in the etch rate, anabrupt step is formed therebetween as described above.

Subsequently, in the procedure shown in FIG. 6( a), the bias plasmaoxidation is carried out for 1 through 5 minutes in an atmosphereincluding oxygen at a substrate temperature of 180° C. and bias power of1800 W. Thus, the entire surfaces of the polysilicon films 18 p and 18 nare oxidized into an oxide film 19 with a thickness of approximately 10nm. This bias plasma oxidation can be carried out at approximately 300°C.

Next, in the procedure shown in FIG. 6( b), the oxide film 19 is removedby etching. As a result, the abrupt step having been present on theboundary between the P-type polysilicon film 18 p and the N-typepolysilicon film 18 n is rounded and disappears.

Then, in the procedure shown in FIG. 6( c), a photoresist film Pr5covering a gate formation region is formed by the photolithography, andthe P-type polysilicon film 18 p and the N-type polysilicon film 18 nare patterned through dry etching using the photoresist film Pr5 as amask, thereby forming a gate electrode 18 a of the PMOSFET and a gateelectrode 18 b of the NMOSFET. At this point, when it is determined thatremoval of the polysilicon films 18 p and 18 n is completed, the gateinsulating films 17 a and 17 b are dotted with etching residues 18 x ofthe polysilicon films 18 p and 18 n. Therefore, with the photoresistfilm Pr5 kept, the bias plasma oxidation is carried out for 5 minutes inan atmosphere including oxygen (or oxygen and nitrogen) at a substratetemperature of 180° C. and bias power of 1000 W, thereby changing theetching residues 18 x into an oxide film (or a nitrided oxide film).

Next, in the procedure shown in FIG. 6( d), after removing thephotoresist film Pr5, portions of the gate insulating films 17 a and 17b not covered with the gate electrode 18 a or 18 b are removed throughthe dry etching. The etching residues 18 x may be oxidized by theaforementioned bias plasma oxidation after removing the photoresist filmPr5.

Subsequently, in the procedure shown in FIG. 7( a), although not shownin the drawing, a photoresist film covering the NMOSFET formation regionRn and having an opening on the PMOSFET formation region Rp is formed,and P-type impurity(ions are implanted by using the photoresist film andthe gate electrode 18 a of the PMOSFET as masks, thereby forming lowconcentration source/drain regions 19 of the PMOSFET. Then, aphotoresist film covering the PMOSFET formation region Rp and having anopening on the NMOSFET formation region Rn is formed, and N-typeimpurity ions are implanted by using the photoresist film and the gateelectrode 18 b of the NMOSFET as masks, thereby forming lowconcentration source/drain regions 20 of the NMOSFET. At this point, itis preferred that the bias plasma oxidation is carried out for forming athin oxide film before forming the photoresist film and that thephotoresist film is removed after the ion implantation.

Next, in the procedure shown in FIG. 7( b), a silicon oxide film isdeposited on the substrate and is then etched back, thereby formingsidewalls 23 a and 23 b on the side faces of the gate electrodes 18 aand 18 b of the MOSFETS. Thereafter, although not shown in the drawing,a photoresist film covering the NMOSFET formation region Rn and havingan opening on the PMOSFET formation region Rp is formed, and P-typeimpurity ions are implanted by using the photoresist film and the gateelectrode 18 a and the oxide film sidewalls 23 a of the PMOSFET asmasks, thereby forming high concentration source/drain regions 21 of thePMOSFET. Then, a photoresist film covering the PMOSFET formation regionRp and having an opening on the NMOSFET formation region Rn is formed,and N-type impurity ions are implanted by using the photoresist film andthe gate electrode 18 b and the oxide film sidewalls 23 b of the NMOSFETas masks, thereby forming high concentration source/drain regions 22 ofthe NMOSFET. At this point, it is preferred that a thin oxide film isformed by the bias plasma oxidation before forming the photoresist film.When a thin oxide film is thus formed, before the ion implantation, bythe plasma oxidation of the gate electrode, the implanted ions can beprevented from punching through the gate electrode.

Then, in the procedure shown in FIG. 7( c), a Co film 24 with athickness of approximately 8 nm is deposited on the substrate, and aheat treatment is conducted at 500° C. for 60 seconds. Thus the Co filmis allowed to react with the Si substrate 11 and the gate electrodes 18a and 18 b in portions where they are in contact with each other,thereby forming a CoSi film (monosilicide film). At this point, theoxide film with a thickness of approximately 1 nm resulting from thebias plasma oxidation may remain on the surfaces of the gate electrodes18 a and 18 b and the Si substrate 11. The remaining oxide film canprevent cobalt from abnormally diffusing along the interface of thepolysilicon, resulting in forming homogenous CoSi₂.

Next, in the procedure shown in FIG. 7( d), an unreacted portion of theCo film 24 is removed, and a heat treatment is carried out at 800° C.for 10 seconds for changing the crystal structure to CoSi₂. Thus,silicide layers 25 a through 25 c with low resistance are formed on thegate electrodes 18 a and 18 b and the high concentration source/drainregions 21 and 22.

In this embodiment, oxide films (or nitrided oxide films) are formedthrough the bias plasma oxidation at various points in the fabricationprocedures for the semiconductor device. As a result, the followingeffects can be attained:

First, since the gate insulating films 17 a and 17 b of oxide films (ornitrided oxide films) are formed by the bias plasma oxidation in theprocedure shown in FIG. 5( a), the gate insulating film 17 a of thePMOSFET and the gate insulating film 17 b of the NMOSFET havesubstantially the same thickness. This is because an oxide film (or anitrided oxide film) formed by the bias plasma oxidation has a thicknessminimally affected by the conductivity type of an impurity introducedinto the underlying Si layer.

FIGS. 8( a) through 8(e) are TEM images of oxide films formed throughthe bias plasma oxidation on a channel region of an NMOSFET (where boronions are implanted), a channel region of a PMOSFET (where arsenic ionsate implanted), a substantially intrinsic substrate (where no ions areimplanted), an N-type polysilicon film (where phosphorus ions areimplanted) and a P-type polysilicon film (where boron ions areimplanted), respectively. As is understood through observation of FIGS.8( a) through 8(c), there is substantially no significant difference inthe thickness among the gate oxide films no matter what kind of ions areimplanted and no matter whether ion implantation is conducted. Adifference in the thickness of approximately 0.6 nm can be regardedwithin a range of measurement error.

In addition, the bias plasma oxidation, which is carried out at a lowtemperature of 180°C., does not change the impurity concentrationprofile in the P-type well region 16 for controlling the thresholdvoltage of the NMOSFET and the impurity concentration profile in theN-type well region 15 for controlling the threshold voltage of thePMOFET. Accordingly, the conventional problems, namely, the accelerationof the short channel effect of the MOSFET, the increase of variation inthe threshold voltages of the NMOSFET and the PMOSFET and the increaseof an off leakage current, can be avoided. Also, an oxide film formed bythe bias plasma oxidation has reliability substantially equivalent tothat of a thermal oxide film.

FIG. 9 is a diagram for showing the result of QBD evaluation of aPMOSFET including, as the gate insulating film, an oxide film formed bythe bias plasma oxidation of this embodiment. FIG. 10 is a diagram forshowing the result of the QBD evaluation of a PMOSFET including, as thegate insulating film, a conventional thermal oxide film(pyrogenicoxidized at 900°C.). As is understood from comparison between FIGS. 9and 10, the oxide film formed by the bias plasma oxidation of thisembodiment and the thermal oxide film formed by the pyrogenic oxidationat 900° C. have substantially equivalent reliability.

Secondly, as is shown in FIG. 4( c), the protection oxide film 13, whosethickness is made ununiform through the ion implantation and the RCAcleaning, is recovered by the bias plasma oxidation into the protectionoxide film 13 a with a uniform thickness before the impurity ions areimplanted for controlling the threshold voltage. Accordingly, thevariation in the threshold voltage of the NMOSFET can be suppressed. Inparticular, this effect of the invention is remarkably exhibited in aPMOSFET because the threshold voltage of a PMOSFET is largely varied inaccordance with change of the impurity concentration.

In a recent semiconductor device designated as a system LSI, such as adevice containing both a DRAM and a logic, a variety of transistors aremounted. Therefore, during the photolithography frequently conducted forion implantation into a variety of transistors, formation and removal ofa photoresist mask is repeated. Accordingly, when an oxide film isrecovered through the bias plasma oxidation every time the RCA cleaningis carried out for removing a photoresist, the threshold values of thevarious transistors can be precisely controlled.

Thirdly, in forming the gate electrodes 18 a and 18 b by patterning thepolysilicon film 18 in the procedure shown in FIGS. 6( c) and 6(d), theetching residues 18 x remaining on the gate insulating films 17 a and 17b and the like are oxidized by the bias plasma oxidation when theetching end point of the polysilicon film is detected. Accordingly, theover-etching can be carried out in removing the gate insulating films 17a and 17 b without causing variation in the etching due to the etchingresidues 18 x. Specifically, even when the gate oxide film has a smallthickness (of several nm), the surface of the active region can besuppressed from being roughened, so that good silicide layers can beformed in the silicidation conducted in the procedures of FIGS. 7( c)and 7(d).

Fourthly, as is shown in FIG. 6( a), the step formed between the P-typepolysilicon film 108 p and the N-type polysilicon film 108 n in removingthe photoresist film Pr4 or in conducting the cleaning before loadingthe substrate in a furnace is rounded by using the oxide film 19 formedby the bias plasma oxidation. Accordingly, the problem of the increasein resistance between the gate electrode 18 a of the PMOSFET and thegate electrode 18 b of the NMOSFET in a CMOS inverter can be solved asfollows:

FIGS. 11( a) through 11(e) are sectional views, in the bias plasmaoxidation and the silicidation, of the structure of the gate electrodes18 a and 18 b alone taken on a section perpendicular to the section ofFIG. 6( a) (namely, a section taken on line XXIV-XXIV of FIG. 25).

The step formed as is shown in FIG. 11( a) on the boundary between theP-type polysilicon film 18 a and the N-type polysilicon film 18 b inremoving the photoresist film Pr4 or in conducting the cleaning beforeloading the substrate in a furnace is rounded by using the oxide film 19formed by the bias plasma oxidation as shown in FIG. 11( b). At thispoint, a heat treatment at a high temperature as in the thermaloxidation is not conducted, and hence, the distributions of theimpurities in the polysilicon films 18 a and 18 b are not harmfullyaffected.

Accordingly, as is shown in FIG. 11( c), there is no abrupt step betweenthe gate electrodes 18 a and 18 b formed by patterning the polysiliconfilm in the procedures shown in FIGS. 6( c) and 6(d). As a result, inthe procedure shown in FIG. 7( c), the Co film 24 with a substantiallyuniform thickness can be deposited on the entire polysilicon filmsforming the gate electrodes 18 a and 18 b as is shown in FIG. 11( d).Furthermore, in the procedure shown in FIG. 7( d), the CoSi₂ films 25 aand 25 b with a substantially uniform thickness can be formed as isshown in FIG. 11( e). Accordingly, when a voltage is applied to the gateelectrode 18 b of the NMOSFET in the CMOS inverter, substantially thesame voltage can be applied to the gate electrode 18 a of the PMOSFET.

FIG. 12( a) is a diagram of the step formed between the N-type andP-type polysilicon films after the ion implantation obtained throughthree-dimensional observation with AFM, and FIG. 12( b) is a diagram forshowing minute steps formed on one section of the N-type and P-typepolysilicon films after the ion implantation. Both FIGS. 12( a) and12(b) show the surface morphology, in the vicinity of the boundarybetween the N-type and P-type polysilicon films after the ionimplantation, changed by the bias plasma oxidation. As is shown in (1)of FIG. 12( a) and shown with a broken line (as reference) of FIG. 12(b), a step or a projection with a level difference of approximately 15nm is observed on the boundary between the N-type polysilicon film andthe P-type polysilicon film before conducting the bias plasma oxidation.As is shown in (2) of FIG. 12( a) and shown with a solid line of FIG.12( b), the level difference of the step or the projection is reduced toapproximately 9 nm by conducting the bias plasma oxidation. In addition,irregularities are rounded as a whole.

FIG. 13 is a diagram for showing a difference in the electric resistancevalue of a polycide film depending upon whether or not the bias plasmaoxidation (BPO) is conducted. In FIG. 13, the abscissa indicates theresistance value (MΩ) and the ordinate indicates the cumulative relativefrequency, whereas measurement data shown at the right end of the graphdo not indicate that the electric resistance values are 1.2 MΩ butindicate that the electric resistance values are large beyond themeasuring range. As is shown in FIG. 13, the resistance is not largelylowered by siliciding polysilicon without the bias plasma oxidation,while the resistance can be lowered by siliciding a polysilicon filmafter the bias plasma oxidation. This supports that the bias plasmaoxidation can change the state of the step formed on the boundarybetween the N-type polysilicon film and the P-type polysilicon film asis schematically shown in FIGS. 24( c) and 11(e).

The polysilicon film can be replaced with an amorphous silicon film. Inthis case, an amorphous silicon film may be subjected to the bias plasmaoxidation even though the amorphous silicon film is changed into apolysilicon film through a heat treatment conducted later.

In the procedure shown in FIG. 5( a), after forming the oxide film bythe bias plasma oxidation, a heat treatment is conducted beforedepositing the polysilicon film for the gate electrodes. Therefore, theoxide film or the like formed by using the bias plasma can attainuniform quality and carbon contamination on the oxide film or the likecan be removed, resulting in improving the reliability of the oxide filmor the like.

Furthermore, in the case where the bias plasma oxidation is carried outin an atmosphere including oxygen and nitrogen, the atmosphere may beobtained by mixing N₂O, N₂, NO or the like with an O₂ gas. When the biasplasma oxidation is carried out in such an atmosphere includingnitrogen, nitrogen can be bonded to an unbonded bond of Si present inthe oxide film or the interface between the Si substrate and the oxidefilm, resulting in forming a nitrided oxide film. In this manner, anitrided oxide film where nitrogen is comparatively uniformlydistributed can be obtained, and hence, when such a nitrided oxide filmis used as a gate insulating film, the function to prevent the variationin the threshold voltage derived from punch-through of the impurity,particularly boron, from the gate electrode into the Si substrate can beadvantageously improved. Among the gases including nitrogen, the NO gasis more preferably used because a NO gas molecule is the smallest sothat a nitrogen atom can be easily introduced into the film. A nitridedoxide film has a function to prevent diffusion of boron, and cansuppress the variation in the threshold voltage.

In a MOSFET having the STI (shallow trench isolation) structure, whenthe gate oxide film is formed by the thermal oxidation, the thickness ofthe gate oxide film is varied depending upon the surface orientation ofthe underlying Si substrate. The Si substrate at the edge of the STI hasthe (111) surface orientation, and the thickness of the oxide film inthis position is small. Therefore, when a voltage is applied to the gateelectrode, the electric field is collected on the edge portion of theSTI where the oxide film is thin, resulting in degrading the reliabilityof the gate oxide film. The thickness of the oxide film formed by thebias plasma oxidation of this invention, however, does not depend uponthe surface orientation of the Si substrate, and hence, the reliabilitythe oxide film is not degraded due to the thinning.

EMBODIMENT 2

FIGS. 14( a) through 14(d) and 15(a) through 15(c) are sectional viewsfor showing procedures in fabrication of a CMOS device having apolymetal gate or polycide gate structure according to Embodiment 2. Inthis embodiment, the procedures after completing the patterning of thegate insulating film and the gate electrodes will be described, and theprevious procedures are omitted. Prior to the procedure of FIG. 14( a),procedures including the bias plasma oxidation as in the procedures ofFIGS. 4( a) through 4(d) of Embodiment 1 may be conducted, or proceduresnot including the bias plasma oxidation as in the conventionalfabrication method may be conducted.

In the procedure shown in FIG. 14( a), a trench isolation region 12 forisolating a PMOSFET formation region Rp and an NMOSFET formation regionRn from each other has been formed in a Si substrate 11. In the PMOSFETformation region Rp, an N-type well region 15, a gate insulating film 17a, a bottom electrode 27 a of polysilicon, a top electrode 28 a of ametal such as tungsten and titanium silicide, and an over-gateprotection film 29 a of silicon oxide are formed, and the bottomelectrode 27 a and the top electrode 28 a together form a gate electrode30 a of the polymetal structure. In the NMOSFET formation region Rn, aP-type well region 16, a gate insulating film 17 b, a bottom electrode27 b of polysilicon, a top electrode 28 b of a metal such as tungstenand titanium silicide, and an over-gate protection film 29 b of siliconoxide are formed, and the bottom electrode 27 b and the top electrode 28b together form a gate electrode 30 b of the polymetal structure. Eachof these gate electrodes 30 a and 30 b of the polymetal structure isformed by successively depositing a polysilicon film, a tungsten film orthe like and a silicon oxide film on the substrate, forming aphotoresist film Pr6 covering a gate formation region by thephotolithography, and conducting dry etching by using the photoresistfilm Pr6 as a mask.

Next, in the procedure shown in FIG. 14( b), with the photoresist filmPr6 kept, the bias plasma oxidation is carried out for 5 minutes in anatmosphere including oxygen (or oxygen and nitrogen) at a substratetemperature of 180° C. and bias power of 1000 W. Thus, a contaminationprotection insulating film 31 of an oxide film (or a nitrided oxidefilm) with a thickness of approximately 6 nm is formed over a surface ofthe Si substrate 11 exposed in the active region and the side faces ofthe bottom electrodes 27 a and 27 b.

Then, in the procedure shown in FIG. 14( c), the photoresist film Pr6 isremoved by the ashing or the RCA cleaning. At this point, the side facesof the top electrodes 28 a and 28 b made from the metal such as tungstenis etched to sink. The surface of the Si substrate 11 in the activeregion is, however, covered with the contamination protection insulatingfilm 31, and hence, metal ions dissolved in the cleaning solution usedin the RCA cleaning can be prevented from entering the Si substrate 11due to the etching of the top electrodes 28 a and 28 b.

Next, in the procedure shown in FIG. 14( d), a photoresist film Pr7covering the NMOSFET formation region Rn and having an opening on thePMOSFET formation region Rp is formed, and P-type impurity ions areimplanted by using the photoresist film Pr7, the gate electrode 30 a ofthe PMOSFET and the like as masks, thereby forming low concentrationsource/drain regions 19 of the PMOSFET. Thereafter, the photoresist filmPr7 is removed by the ashing and the RCA cleaning. At this point, sincethe contamination protection insulating film 31 is also etched by theRCA cleaning, the bias plasma oxidation is preferably carried out afterremoving the photoresist film Pr7, so as to recover the thickness of thecontamination protection insulating film 31.

Subsequently, in the procedure shown in FIG. 15( a), a photoresist filmPr8 covering the PMOSFET formation region Rp and having an opening onthe NMOSFET formation region Rn is formed, and N-type impurity ions areimplanted by using the photoresist film Pr8, the gate electrode 30 b ofthe NMOSFET and the like as masks, thereby forming low concentrationsource/drain regions 20 of the NMOSFET. Thereafter, the photoresist filmPr8 is removed by the ashing and the RCA cleaning. At this point, sincethe contamination protection insulating film 31 is also etched by theRCA cleaning, the bias plasma oxidation is preferably carried out afterremoving the photoresist film Pr8, so as to recover the thickness of thecontamination protection insulating film 31.

Next, in the procedure shown in FIG. 15( b), a silicon oxide film 32 isdeposited on the substrate, and in the procedure shown in FIG. 15( c),the silicon oxide film 32 is etched back, thereby forming oxide filmsidewalls 32 a and 32 b on the side faces of the over-gate protectionfilms 29 a and 29 b and the gate electrodes 30 a and 30 b of theMOSFETs. Thereafter, although not shown in the drawing, a photoresistfilm covering the NMOSFET formation region Rn and having an opening onthe PMOFET formation region Rp is formed, and P-type impurity ions areimplanted by using the photoresist film and the gate electrode 30 a andthe oxide film sidewall 32 a of the PMOSFET as masks, thereby forminghigh concentration source/drain regions 21 of the PMOSFET. Subsequently,a photoresist film covering the PMOSFET formation region Rp and havingan opening on the NMOSFET formation region Rn is formed, and N-typeimpurity ions are implanted by using the photoresist film and the gateelectrode 30 b and the oxide film sidewall 32 b of the NMOSFET as masks,thereby forming high concentration source/drain regions 22 of theNMOSFET.

Thereafter, after removing the contamination protection insulating film31 on the high concentration source/drain regions 21 and 22, silicidefilms may be formed on the high concentration source/drain regions 21and 22 by depositing a refractory metal film of Co or the like andcausing a reaction between the refractory metal and the Si substrate 11.

In the fabrication method of this embodiment, before removing thephotoresist film Pr6 by the ashing and the RCA cleaning, thecontamination protection insulating film 31 is formed by the bias plasmaoxidation with the photoresist film Pr6 kept as is shown in FIG. 14( c).Therefore, even when the top electrodes 28 a and 28 b are etched inremoving the photoresist film Pr6 thereafter in the procedure of FIG.14( c), the metal ions included in the top electrodes 28 a and 28 b canbe suppressed from entering the Si substrate 11. Accordingly, junctionleakage derived from the metal ions entering the Si substrate 11 can beeffectively suppressed. As a significant characteristic of the biasplasma oxidation conducted at this point, there is no need to remove thephotoresist film Pr6 because this bias plasma oxidation is carried outat a low temperature of 200° C. or less. In other words, thecontamination protection insulating film 31 for protecting the surfaceof the Si substrate 11 can be formed without generating debrisaccompanied by the removal of the photoresist film Pr6.

Also, since oxidation is conducted at a low temperature by using thebias plasma oxidation, the source/drain regions can be prevented frombeing etched by the RCA cleaning for removing the photoresist film, andthus, the sheet resistance is definitely prevented from increasing.

In addition, since the bias plasma oxidation is carried out at a lowtemperature of 200° C. or less, the top electrodes 28 a and 28 b madefrom the metal are never oxidized.

Furthermore, the bias plasma oxidation carried out at a low temperaturehas another advantage that the junction profile formed before the biasplasma oxidation by the ion implantation for forming a well region orthe like is never changed.

The invention is applicable to fabrication of all kinds of semiconductordevices having a gate electrode including a metal, such as agateelectrode of the polymetal structure, a gate electrode of the polycidestructure, and a gate electrode having a metal structure.

Now, a modification of Embodiment 2 will be described. In thismodification, a nitride film sidewall is formed instead of the oxidefilm sidewall.

FIG. 16 is a sectional view of a CMOS device fabricated in thismodification. In this modification, the PMOSFET and the NMOSFET areformed through the basically same procedures as those shown in FIGS. 14(a) through 14(d) and 15(a) through 15(c), whereas over-gate protectionfilms 51 a and 51 b of a nitride film are formed instead of theover-gate protection films 29 a and 29 b of the oxide film and nitridefilm sidewalls 52 a and 52 b are formed instead of the oxide filmsidewalls 32 a and 32 b. The other elements are formed through the sameprocedures as those shown in FIGS. 14( a) through 14(d) and 15(a)through 15(c).

In this modification, after the procedure shown in FIG. 15( c), aninterlayer insulating film 55 of a BPSG film is deposited on thesubstrate, contact holes respectively in contact with the highconcentration source/drain regions 21 and 22 are formed through theinterlayer insulating film 55, and a plug 56 is formed by filling eachcontact hole with a tungsten film with a barrier film of titaniumnitride or the like sandwiched therebetween. Furthermore, a line layer57 of an aluminum alloy film connected to each plug 56 is formed on theinterlayer insulating film 55.

At this point, owing to the over-gate protection films 51 a and 51 b ofthe nitride film and the nitride film sidewalls 52 a and 52 b, eachcontact hole never reaches the gate electrode 30 a or 30 b even if it isformed in a region overlapping the gate electrode 30 a or 30 b. In otherwords, there is no need to provide a mask for forming the contact holewith a margin in consideration of alignment shift against a mask forforming a gate electrode, but the so-called SAC (self-aligned contact)process can be applied.

In this modification, a contamination protection film 31 is formedmerely on the side faces of the bottom electrodes 27 a and 27 b and thetop face of the Si substrate 11 by the bias plasma oxidation withoutharmfully affecting the characteristic of the top electrodes 28 a and 28b of the metal film as is shown in FIG. 14( b). Thus, although thenitride film sidewalls 52 a and 52 b are formed thereon, the applicationof stress to the channel region or the like due to the nitride film canbe definitely suppressed. Accordingly, even though the nitride filmsidewalls 52 a and 52 b are formed, the electric characteristics of thePMOSFET and the NMOSFET can be satisfactorily kept.

In a know fabrication method for a CMOS device having a gate electrodeof the polymetal structure or the polycide structure, a nitride filmsidewall is formed on the side faces of the gate electrode and theover-gate protection film of a nitride film with an oxide film sidewallsandwiched therebetween. In this conventional fabrication method,however, the following problem arises in forming a contact in aself-alignment manner: When a contact hole is formed over the gateelectrode, the upper portion of the nitride film sidewall with a verysmall thickness can be easily etched, and the oxide film sidewall can beetched in that portion, so that the contact hole can reach the gateelectrode (top electrode). Accordingly, in order to definitely preventelectric short-circuit between the source/drain region and the gateelectrode, the SAC process is difficult to employ in the conventionalmethod.

In contrast, in the CMOS device formed by the fabrication method of thismodification, the over-gate protection film 51 a or 51 b of the nitridefilm is in contact with the nitride film sidewall 52 a or 52 b in aportion corresponding to the thickness of the over-gate protection film51 a or 51 b. Therefore, even when a contact hole is formed over thegate electrode, the contact hole can be definitely prevented fromreaching the gate electrode. In addition, the nitride film sidewalls 52a and 52 b can be formed in a small thickness, and hence, the MOSFETscan be easily refined. In other words, this fabrication method providesa CMOS device having the polymetal gate or polycide structure and theSAC structure, with advantage in refinement.

EMBODIMENT 3

FIGS. 17( a) through 17(c) and 18(a) through 18(c) are sectional viewsfor showing procedures in fabrication of a CMOS device having thesalicide structure according to Embodiment 3. In this embodiment,procedures after completing the patterning of the gate insulating filmand the gate electrodes will be described, and the previous proceduresare omitted. Prior to the procedure shown in FIG. 17( a), proceduresincluding the bias plasma oxidation as in those of Embodiment 1 shown inFIGS. 4( a) through 4(d) may be conducted, or procedures not includingthe bias plasma oxidation of this invention as in the conventionalfabrication method may be conducted.

In the procedure shown in FIG. 17( a), a trench isolation region 12 forisolating the PMOSFET formation region Rp and the NMOSFET formationregion Rn from each other has been formed in a Si substrate 11. In thePMOSFET formation region Rp, an N-type well region 15, a gate insulatingfilm 17 a and a gate electrode 18 a of polysilicon are formed. In theNMOSFET formation region Rn, a P-type well region 16, a gate insulatingfilm 17 b and a gate electrode 18 b of polysilicon are formed.

Next, in the procedure shown in FIG. 17( b), the bias plasma oxidationis carried out for 5 minutes in an atmosphere including oxygen at asubstrate temperature of 180° C. and bias power of 1000 W, therebyforming a coat oxide film 35 with a thickness of approximately 6 nm overa surface of the Si substrate 11 exposed in the active region and theside and top faces of the gate electrodes 18 a and 18 b. This biasplasma oxidation can be carried out at approximately 300°C.

Then, in the procedure shown in FIG. 17( c), a photoresist film Pr9covering the NMOSFET formation region Rn and having an opening on thePMOSFET formation region Rp is formed, and boron fluoride (BF₂ ⁺) (orboron (B⁺)) ions are implanted by using the photoresist film Pr9 and thegate electrode 18 a of the PMOSFET as masks at implantation energy of 8keV and a dose of 1×10¹³ through 1×10¹⁴ cm⁻², thereby forming lowconcentration source/drain regions 19 of the PMOSFET. Thereafter, thephotoresist film Pr9 is removed by the ashing and the RCA cleaning. Theremoval of the photoresist film Pr9 by the ashing is conducted as aprocess for applying plasma in an oxygen gas atmosphere, namely, in asimilar process to the bias plasma oxidation, but the bias power islowered in the ashing than in the bias plasma oxidation conducted in theprocedure of FIG. 17( b). In this manner, the coat insulating film 35formed in the procedure of FIG. 17( c) can be prevented from increasingin its thickness through oxidation. In other words, the impurity profilecan be prevented from being changed by the oxide film abrading the lowconcentration source/drain regions 19 formed in the Si substrate 11, andhence, the change or degradation of the electric characteristic of theMOSFET derived from the change of the impurity profile can be avoided.

In the RCA cleaning following the ashing, the low concentrationsource/drain regions 19 are not etched because they are covered with thecoat insulating film 35, resulting in preventing the resistance of thelow concentration source/drain regions 19 from increasing.

Subsequently, in the procedure shown in FIG. 18( a), a photoresist filmPr10 covering the PMOSFET formation region Rp and having an opening onthe NMOSFET formation region Rn is formed, and arsenic (As⁺) (orphosphorus (P⁺)) ions are implanted by using the photoresist film Pr10and the gate electrode 18 b of the NMOSFET as masks at implantationenergy of 10 keV and a dose of 1×10¹³ through 1×10¹⁴ cm⁻², therebyforming low concentration source/drain regions 20 of the NMOSFET.Thereafter, the photoresist film Pr10 is removed by the ashing and theRCA cleaning. Also in removing the photoresist film Pr10 by the ashing,the bias power is lowered than in the bias plasma oxidation conducted inthe procedure of FIG. 17( b). In this manner, the coat insulating film35 is prevented from increasing in its thickness by the oxidation, sothat the change or degradation of the electric characteristic of theMOSFET derived from the change of the impurity profile in the lowconcentration source/drain regions 20 can be avoided.

Thereafter, before loading the substrate in an electric furnace, the RCAcleaning is carried out again for removing particles. However, afterremoving the photoresist film Pr10, the bias plasma oxidation ispreferably carried out before the cleaning for removing particles so asto recover the thickness of the coat oxide film 35. This is because thecoat oxide film 35 is etched in the RCA cleaning.

Next, in the procedure shown in FIG. 18( b), after the cleaning forremoving particles, the coat oxide film 35 is removed, and a siliconoxide film 32 is deposited on the substrate. In conducting the RCAcleaning for removing particles, if the coat oxide film scarcely remainson the semiconductor active region, a bias plasma insulating film isformed before the cleaning.

Then, in the procedure shown in FIG. 18( c), the silicon oxide film 32is etched back, thereby forming oxide film sidewalls 32 a and 32 b onthe side faces of the gate electrodes 18 a and 18 b of the MOSFETs.Thereafter, although not shown in the drawing, a photoresist filmcovering the NMOSFET formation region Rn and having an opening on thePMOSFET formation region Rp is formed, and P-type impurity ions areimplanted by using the photoresist film and the gate electrode 18 a andthe oxide film sidewall 32 a of the PMOSFET as masks, thereby forminghigh concentration source/drain regions 21 of the PMOSFET. Subsequently,a photoresist film covering the PMOSFET formation region Rp and havingan opening on the NMOSFET formation region Rn is formed, and N-typeimpurity ions are implanted by using the photoresist film and the gateelectrode 18 b and the oxide film sidewall 32 b of the NMOSFET as masks,thereby forming high concentration source/drain regions 22 of theNMOSFET.

Thereafter, the salicidation is conducted, which is omitted because itis the same as that described in Embodiment 1.

In the fabrication method of this embodiment, in forming the coatinsulating film 35 for covering the gate electrodes 18 a and 18 b in theprocedure shown in FIG. 17( b), the oxidation is conducted at a lowtemperature by using the bias plasma oxidation. Accordingly, the sheetresistance can be avoided from increasing due to the source/drain regionetched by the RCA cleaning for removing the photoresist film.

In addition, the boron can be prevented from diffusing from the gateelectrode 18 a of the PMOSFET through the gate oxide film 17 a into theN-type well region 15 as in a heat treatment conducted at 900 through1000° C. for forming an oxide film by the conventional thermaloxidation. Also, in the STI structure, displacement derived from a hightemperature heat treatment can be prevented from occurring in thesubstrate. In this manner, the degradation in the reliability and theincrease of the variation in the threshold voltage of the PMOSFET can beprevented. Also, the change of the impurity profile in the substrate canbe suppressed.

Furthermore, the ashing for removing the photoresist films Pr9 and Pr10of FIGS. 17( c) and 18(a) is carried out at bias power lower than in thebias plasma oxidation conducted in the procedure of FIG. 17( b).Therefore, the coat insulating film 35 formed in the procedure of FIG.17( c) can be prevented from increasing in its thickness by theoxidation. In other words, it is possible to overcome a problem that aportion with a peak concentration in the low concentration source/drainregion 19 or 20 is changed into an oxide film because of the oxide filmeroding the low concentration source/drain region 19 or 20 formed in theSi substrate 11. Therefore, the change or degradation of the electriccharacteristic of the MOSFET derived from the change of the impurityprofile can be avoided.

FIGS. 19( a) and 19(b) are diagrams for showing the Ion-Ioffcharacteristics of the PMOSFET and the NMOSFET formed in thisembodiment. In FIGS. 19( a) and 19(b), the abscissa indicates asaturation current Ion (μA) of the MOSFET and the ordinate indicates anoff leakage current Ioff. In these drawings, data of a MOSFET as acomparative example where a coat insulating film is not formed by thebias plasma oxidation are shown with white circles, and data of theMOSFET of this embodiment where the coat insulating film is formed bythe bias plasma oxidation are shown with black triangles. It isunderstood from FIGS. 19( a) and 19(b) that the off leakage current isfurther lowered against the same saturation current in the MOSFET ofthis embodiment than in the MOSFET of the comparative example. In otherwords, the so-called off-leakage characteristic is improved in theMOSFET of this embodiment.

EMBODIMENT 4

FIGS. 20( a) through 20(c) are sectional views for showing part ofprocedures for forming a trench isolation region of a semiconductordevice according to Embodiment 4.

In the procedure shown in FIG. 20( a), after depositing a pad oxide film41 and a masking nitride film 42 on a Si substrate 11, a photoresistfilm (not shown) having an opening on an area where a trench is to beformed is formed by the photolithography, so as to etch a portion of themasking nitride film 42 above the area where the trench is to be formedby using the photoresist film as a mask. Then, the photoresist film isremoved, and dry etching is carried out by using the masking nitridefilm 42 as a mask, thereby forming a trench 43 in the Si substrate 11.Since portions of the pad oxide film 44 exposed on the sidewalls of thetrench 43 are etched by the RCA cleaning and the etching with dilutedhydrofluoric acid (BHF) conducted thereafter, the pad oxide film 41sinks to form a gap 44.

Next, in the procedure shown in FIG. 20( b), the bias plasma oxidationis carried out for 5 minutes by using the plasma generation system shownin FIG. 1 in an atmosphere including oxygen at a substrate temperatureof 180° C. and bias power of 1000 W. Through this bias plasma oxidation,an electric field relaxing oxide film 45 having a thickness of 10 nmwithin the trench is formed. An abrupt upper edge of the Si substrate 11exposed within the trench 43 is rounded by this electric field relaxingoxide film 45, so as to suppress the breakdown of a gate insulating filmand the hump characteristic derived from electric field collection of aMOSFET formed therein.

Then, in the procedure shown in FIG. 20( a), a CVD oxide film isdeposited on the entire substrate and is then etched back, therebyfilling the CVD oxide film within the trench. Thus, a trench isolationregion 46 is formed.

Since the electric field relaxing oxide film 45 is formed by the biasplasma oxidation in this embodiment, it is not affected by the shapes ofthe pad oxide film and the masking nitride film differently from anoxide film obtained by the conventional thermal oxidation, and hence,the edge can be changed into a good round shape for relaxing theelectric field. Furthermore, since the electric field relaxing oxidefilm 45 is formed by the bias plasma oxidation conducted at a lowtemperature of 200° C. or less, the stress derived from the formation ofthe oxide film 45 can be suppressed from occurring. Accordingly, defectssuch as displacement in the Si substrate 11 can be prevented fromoccurring.

Alternatively, additional oxidation may be carried out by the thermaloxidation after forming the electric field relaxing oxide film 45 by thebias plasma oxidation before filling the trench with a CVD oxide film.Also in this case, the edge of the Si substrate 11 within the trench isrounded by the bias plasma oxidation, and hence, the additionaloxidation by using the thermal oxidation never causes the honephenomenon.

In each of the embodiments, the description has been given on merelyoxidation (or nitriding oxidation) of a surface portion of a Si layer bythe bias plasma oxidation, which does not limit the invention. Forexample, any of various semiconductor layers, such as a Ge layer, a SiGelayer, a SiGeC layer, a GaAs layer and an AlGaAs layer, can be subjectedto the bias plasma oxidation to form an oxide or nitrided oxide film.Also in this case, the same effects as those attained in the embodimentscan be exhibited.

Furthermore, a plasma oxide film may be formed after forming an STIgroove in a SOI substrate.

1. A method of fabricating a semiconductor device comprising the stepsof: (a) successively depositing a first insulating film and a conductingfilm at least including a metal on a semiconductor substrate; (b)patterning said conducting film and said first insulating film byetching with a photoresist film used as a mask into a gate electrode anda gate insulating film; (c) forming a second insulating film on at leastan exposed portion of said semiconductor substrate through a reactionbetween oxygen and a semiconductor by subjecting, in an atmosphereincluding oxygen, said semiconductor substrate to plasma biased towardsaid semiconductor substrate, with said photoresist film kept; (d)removing said photoresist film; and (e) forming source/drain regions byintroducing an impurity into regions positioned on both sides of saidgate electrode in said semiconductor substrate.
 2. The method offabricating a semiconductor device of claim 1, wherein a polysiliconfilm and a metal film stacked thereon are formed as said conducting filmin the step (a), a bottom electrode of a polysilicon film and a topelectrode of a metal film are formed as said gate electrode in the step(b), and said second insulating film is formed also on side faces ofsaid bottom electrode in the step (c).
 3. The method of fabricating asemiconductor device of claim 2, wherein a silicon nitride film isfurther formed on said conducting film in the step (a), an over-gateprotection film of a nitride film is formed on said top electrode in thestep (b), and the method further includes, after the step (d), steps of:(f) forming nitride film sidewalls on side faces of said polysiliconfilm and said metal film; (g) depositing an interlayer insulating filmof a silicon oxide film on said substrate; and (h) forming a contacthole penetrating through said interlayer insulating film and reachingsaid source/drain region in a self alignment manner against said gateelectrode.
 4. The method of fabricating a semiconductor device of claim1, wherein the step (c) is carried out at a temperature of 200° or less.